A fair percentage is whatever you need to cover your expenses. You have time invested to pick up the materials, drive back to the job and provide a warranty as well as waiting for your money until you get paid.
If you think things will go bad by charging too much, consider what will happen to your business if you charge too little.
A sliding scale is used by some based on the item cost. For example you can get a higher percentage markup on receptacles than you can on switchgear.
Are you buying these materials just for this job or are they truck stock? Will these parts be able to be used on other jobs?
